/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.messaging.support;

import org.springframework.lang.Nullable;
import org.springframework.messaging.Message;
import org.springframework.messaging.MessageDeliveryException;
import org.springframework.messaging.MessageHandler;
import org.springframework.messaging.MessagingException;
import org.springframework.messaging.SubscribableChannel;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Executor;

/**
 * A {@link SubscribableChannel} that sends messages to each of its subscribers.
 *
 * @author Phillip Webb
 * @author Rossen Stoyanchev
 * @since 4.0
 */
public class ExecutorSubscribableChannel extends AbstractSubscribableChannel {

    @Nullable
    private final Executor executor;

    private final List<ExecutorChannelInterceptor> executorInterceptors = new ArrayList<>(4);


    /**
     * Create a new {@link ExecutorSubscribableChannel} instance
     * where messages will be sent in the callers thread.
     */
    public ExecutorSubscribableChannel() {
        this(null);
    }

    /**
     * Create a new {@link ExecutorSubscribableChannel} instance
     * where messages will be sent via the specified executor.
     *
     * @param executor the executor used to send the message,
     *                 or {@code null} to execute in the callers thread.
     */
    public ExecutorSubscribableChannel(@Nullable Executor executor) {
        this.executor = executor;
    }


    @Nullable
    public Executor getExecutor() {
        return this.executor;
    }

    @Override
    public void setInterceptors(List<ChannelInterceptor> interceptors) {
        super.setInterceptors(interceptors);
        this.executorInterceptors.clear();
        interceptors.forEach(this::updateExecutorInterceptorsFor);
    }

    @Override
    public void addInterceptor(ChannelInterceptor interceptor) {
        super.addInterceptor(interceptor);
        updateExecutorInterceptorsFor(interceptor);
    }

    @Override
    public void addInterceptor(int index, ChannelInterceptor interceptor) {
        super.addInterceptor(index, interceptor);
        updateExecutorInterceptorsFor(interceptor);
    }

    private void updateExecutorInterceptorsFor(ChannelInterceptor interceptor) {
        if (interceptor instanceof ExecutorChannelInterceptor) {
            this.executorInterceptors.add((ExecutorChannelInterceptor) interceptor);
        }
    }


    @Override
    public boolean sendInternal(Message<?> message, long timeout) {
        for (MessageHandler handler : getSubscribers()) {
            SendTask sendTask = new SendTask(message, handler);
            if (this.executor == null) {
                sendTask.run();
            } else {
                this.executor.execute(sendTask);
            }
        }
        return true;
    }


    /**
     * Invoke a MessageHandler with ExecutorChannelInterceptors.
     */
    private class SendTask implements MessageHandlingRunnable {

        private final Message<?> inputMessage;

        private final MessageHandler messageHandler;

        private int interceptorIndex = -1;

        public SendTask(Message<?> message, MessageHandler messageHandler) {
            this.inputMessage = message;
            this.messageHandler = messageHandler;
        }

        @Override
        public Message<?> getMessage() {
            return this.inputMessage;
        }

        @Override
        public MessageHandler getMessageHandler() {
            return this.messageHandler;
        }

        @Override
        public void run() {
            Message<?> message = this.inputMessage;
            try {
                message = applyBeforeHandle(message);
                if (message == null) {
                    return;
                }
                this.messageHandler.handleMessage(message);
                triggerAfterMessageHandled(message, null);
            } catch (Exception ex) {
                triggerAfterMessageHandled(message, ex);
                if (ex instanceof MessagingException) {
                    throw (MessagingException) ex;
                }
                String description = "Failed to handle " + message + " to " + this + " in " + this.messageHandler;
                throw new MessageDeliveryException(message, description, ex);
            } catch (Throwable err) {
                String description = "Failed to handle " + message + " to " + this + " in " + this.messageHandler;
                MessageDeliveryException ex2 = new MessageDeliveryException(message, description, err);
                triggerAfterMessageHandled(message, ex2);
                throw ex2;
            }
        }

        @Nullable
        private Message<?> applyBeforeHandle(Message<?> message) {
            Message<?> messageToUse = message;
            for (ExecutorChannelInterceptor interceptor : executorInterceptors) {
                messageToUse = interceptor.beforeHandle(messageToUse, ExecutorSubscribableChannel.this, this.messageHandler);
                if (messageToUse == null) {
                    String name = interceptor.getClass().getSimpleName();
                    if (logger.isDebugEnabled()) {
                        logger.debug(name + " returned null from beforeHandle, i.e. precluding the send.");
                    }
                    triggerAfterMessageHandled(message, null);
                    return null;
                }
                this.interceptorIndex++;
            }
            return messageToUse;
        }

        private void triggerAfterMessageHandled(Message<?> message, @Nullable Exception ex) {
            for (int i = this.interceptorIndex; i >= 0; i--) {
                ExecutorChannelInterceptor interceptor = executorInterceptors.get(i);
                try {
                    interceptor.afterMessageHandled(message, ExecutorSubscribableChannel.this, this.messageHandler, ex);
                } catch (Throwable ex2) {
                    logger.error("Exception from afterMessageHandled in " + interceptor, ex2);
                }
            }
        }
    }

}
